Variable resistor with linearly movable means for changing the resistance thereof

ABSTRACT

A variable resistor unit is provided which comprises a resistance element arranged in an annular configuration and a contact arm turnable on a shaft in sliding contact with the resistance element. The variable resistor unit with its shaft is supported by a support plate which is movable linearly as directed by guide means including a guide rod. A pulley is fixedly mounted on the shaft of the variable resistor unit, and a string wrapped around the pulley extends crosswise therefrom to have its ends tightly retained at both extremities of the stroke of the support plate. Hence, as the support plate is moved linearly by means of a knob affixed thereto, the pulley is turned by the string to cause the corresponding change in the angular position of the contact arm relative to the annular resistance element of the variable resistor unit.

United States Patent [191' Murao et al.

[ 1 VARIABLE RESISTOR WITH LINEARLY MOVABLE MEANS FOR CHANGING THERESISTANCE THEREOF [75] Inventors: Kunihiko Murao; Tsutomu [52] US. Cl338/162, 338/150, 338/176,

. 338/197 [51] Int. Cl HOIc 9/02 [58] Field of Search 338/150, 152, 160,162,

[ Mar. 26, 1974 Primary Examiner-Bernard A. Gilheany AssistantExaminer-David A. Tone Attorney, Agent, or Firm-Woodcock, Washburn,Kurtz & Mackiewicz [57] ABSTRACT A variable resistor unit is providedwhich comprises a resistance element arranged in an annularconfiguration and a contact arm turnable on a shaft in sliding contactwith the resistance element. The variable resistor unit with its shaftis supported by a support plate which is movable linearly as directed byguide means including a guide rod. A pulley is fixedly mounted on theshaft of the variable resistor unit, and a string wrapped around thepulley extends crosswise therefrom to have its ends tightly retained atboth extremities of the stroke of the support plate. Hence, as thesupport plate is moved linearly by means of a knob affixed thereto, thepulley is turned by the string to cause the corresponding change in theangular position of the contact arm relative to the annular resis tanceelement of the variable resistor unit.

12 Claims, 11 Drawing Figures [56] References Cited UNITED STATESPATENTS 2,492,727 12/1949 Ballard 338/174 X 3,184,696 5/1965 Snyder338/174 3,493,914 2/1970 Campbell 338/184 FOREIGN PATENTS ORAPPLlCATlONS 871,023 7/1949 Germany 338/183 11 24 10 l i L r l 13a 22a"s J MENIEDMAR2S 1914 SHEET 1 (IF 3 FIG.2

FIGA

FIG.3

PAIENIEUmzs m4 SHiET 3 BF 3 FIG."

VARIABLE RESISTOR WITH LINEARLY MOVABLE MEANS FOR CHANGING THERESISTANCE THEREOF BACKGROUND OF THE INVENTION This invention relates tovariable resistors, to volume controls, and to improved means forchanging the resistance of a variable resistor of the rotary type by thelinear motion of a knob or the like.

It is well known that the volume controls as heretofore used in variousaudio equipment of the like have usually been composed of a variableresistor of the rotary type including an annular resistance element anda contact arm mounted on a shaft. In order to change the resistance ofsuch a rotary variable resistor, a knob affixed to the shaft has beenmanually revolved to cause the contact arm to turn in sliding contactwith the resistance element.

More recently, however, there have been increasingly used the variableresistors comprising a linearly arranged resistance element and acontact arm which is movable linearly in sliding contact with the formerto vary the resistance thereof. This latter type of volume they can bemanipulated.

There is one serious disadvantage with the linear type of variableresistors, however, in that their resistance elements cannot possibly beenclosed in a dustproof housing. The accumulation of dust on theresistance elements results in the production of noise. Furthermore, thelinearly arranged resistance elements tend to become detached, at leastin part, from their insulating bases as the contact arms are movedrepeatedly in sliding contact therewith. This can also seriously affectthe operation of the variable resistors.

It should also be taken into consideration that for the fine adjustmentof the resistance offered by the variable resistors of the linear type,the stroke of the knob and therefore of the effective size of theirresistance elements must be considerably increased. However, theproduction of linear resistance elements of such increased size havingunfluctuating resistance distribu tion is admittedly a very difficulttask.

SUMMARY OF THE INVENTION In view of the listed disadvantages anddifficulties of the prior art, it is an object of this invention toprovide a novel combination of a variable resistor of the rotary typeand means for changing the resistance thereof by the linear motion of aknob or the like, such that the novel combination is particularly wellsuited for use as a volume control in various audio equipment or thelike.

Another object of the invention is to provide apparatus of the characterreferred to, wherein the variable resistor in use can be completelyenclosed in a dustproof housing so that its service life is greatlyextended.

A further object of the invention is to provide apparatus of thecharacter referred to, such that the fine adjustment of the resistanceoffered by the variable resistor can easily be made possible by suitablyincreasing the diameter of a pulley adapted to translate the linearmotion of the knob into the rotary motion of a contact arm in slidingcontact with an annularly arranged resistance element of the variableresistor.

A further object of the invention is to provide apparatus of thecharacter described, wherein a string is wrapped around the pulley insuch a fashion that the linear motion of the knob is faithfullyconverted into its rotation.

A still further object of the invention is to provide apparatus of thecharacter described, such that the knob can be moved linearly in eitherdirection with minimum frictional resistance.

It is also an object of this invention to provide apparatus of thecharacter referred to, which is positive in operation, simple andinexpensive in construction, and extremely easy to assemble.

With these objects in view and other objects hereinafter made apparent,the present invention provides the combination including a variableresistor unit of the type having a resistance element arranged in asubstantially annular configuration, a turnable contact arm, and a shaftsupporting the contact arm. The variable resistor unit is supported bysupport means in such a manner that its shaft is turnable relative tothe support means to cause the contact arm of the variable resistor unitto turn in sliding contact with the resistance element. The supportmeans together with the variable resistor unit is linearly movable overa predetermined stroke as directed by guide means including a guide rod.A pulley is fixedly mounted on the shaft of the variable resistor unitfor linear motion with the support means, and astring wrapped round thepulley extends crosswise therefrom to have its ends affixed at bothextremities of the stroke of the support means. As the support meanstogether with the variable resistor unit is moved linearly by means ofaknob provided thereto, the pulley is turned by the string through anangle in proportion to the distance of travel of the support meansthereby causing the corresponding change in the angular position of thecontact arm relative to the resistance element through the shaft of thevariable resistor unit. The guide means may include means adapted tominimize the frictional resistance offered to the linear motion of thesupport means.

The features which are believed to be novel and'characteristic of thisinvention are set forth with particularity in the appended claims. Theinvention itself, however, both as to its construction and mode ofoperation, together with the further objects and advantages thereof,will become more apparent and understandable as the descriptionproceeds, with reference taken to the accompanying drawings whichillustrate, by way of example only, some preferred embodiments of theinvention and in which like reference characters designate like partsthroughout the several views.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings:

FIG. 1 is a top view showing a knob movable on a slitted panel to changethe resistance of a variable resistor according to this invention;

FIG. 2 is a front elevational view of a first preferred embodiment ofthe invention;

FIG. 3 is a view taken along the plane of line IlI--III in FIG. 2;

FIG. 4 is a side elevational view, partly in axial section, of avariable resistor unit incorporated in the embodiment of FIGS. 1 and 2;

FIG. 5 is an enlarged schematic view showing the internal constructionof the variable resistor unit of FIG.

FIG. 6 is a front elevational view of a second preferred embodiment ofthe invention;

FIG. 7 is a view taken along the plane of line VII- -VII in FIG. 6.

FIG. 8 is a fragmentary back elevational view, partly in axial section,of a support plate and its guide rod in the embodiment of FIGS. 7 and 8;

FIG. 9 is a fragmentary side elevational view of the means shown in FIG.8;.

FIG. 10 is an enlarged perspective view ofa slide cylinder employed tominimize the frictional resistance between the support plate and theguide rod best illustrated in FIG. 8 and 9; and

FIG. 11 is an enlarged, fragmentary, exploded per spective view showinga modified example of a string adapted to translate the linear motion ofthe knob into the rotary motion of a pulley in the embodiment of FIGS. 6and 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A first preferred embodiment ofthis invention will now be described in detail with reference to FIGS. 1to 5. Referring first to FIGS. 1 and 2, there is provided an elongatepanel 10 having a slit 11 extending longitudinally thereof. A supportplate 12 of substantially rectangular shape provided below the panel 10has its tongue 12a rigidly coupled to a knob 13 through the slit 11. Apair of terminal members 13a and 13b are secured to the lower surface ofthe panel 10 at or adjacent both extremities of the slit 1] formedtherein. Affixed at 14 to one of the surfaces of the support plate 12 isa variable resistor unit of the rotary type having a shaft 15 projectingoutwardly therefrom to the other side of the support plate 12. Thevariable resistor unit 14 may include a dustproof housing enclosingvarious operative parts thereof as in the drawings.

As best shown in FIG. 3, a pulley 16 is screwed as at 17 to the shaft 15of the variable resistor unit 14 on the said other side of the supportplate 12. This pulley has a pair of parallel grooves 180 and 18b formedcircumferentially thereof to. accommodate a cord or string 19 which maybe formed of wires, nylon fibers, or the like or which may be providedin the form of a belt of appropriately small width. The string 19 iswrapped a single turn around the pulley 16, crossing from one of theparallel grooves 18a and 18b to the other through an indentation 20formed in the raised border herebetween. If desired, the string 19 maybe affixed to the pulley 16 at the indentation 20 as by means of a screw21, although this is not absolute necessity. Upon completely encirclingthe pulley 16 in this manner, the string 19 further extends crosswisesubstantially tangentially thereof to have its end tightly affixed tothe terminal members 13a and 13b, respectively. In the illustratedembodiment of the invention the ends of the string 19 are inserted intobores 22a and 22b, respectively, of the members 13a and 13b and areunloosenably retained by screws 23a and 23b, respectively.

In order to assure the linear motion of the support plate 12 and theknob 13, a guide rod 24 extends between the terminal members 13a and 13bthrough a bore 25 formed in a member 26 secured to the support plate 12as by means of screws 27.

As illustrated in detail in FIGS. 4 and 5, the shaft 15 of the variableresistor unit 14 rotatably supported by a bearing block 28 which isexternally screw threaded to mesh with the corresponding internalthreads of a nut 29 adapted to hold the variable resistor unit securelyin position on the support plate 12. The variable resistor unit is ofsuch construction that its shaft 15 is fixedly provided with a contactarm 30 which as the shaft 15 is revolved in a manner hereinafter setforth, turns in sliding contact with a resistance element 31 arranged ina substantially annular configuration on an electrically insulating base32 and having terminals 33 and 34 connected to both extremities thereof.Thus, with the change in the angular position of the movable contact arm30 relative to the resistance element 31, the electrical resistancebetween the terminal 33 and a terminal '35 electrically connected to thecontact arm 30 and that between the terminal 35 and the other terminal34 are varied as desired.

Proceeding now to the description of a mode of operation of the firstpreferred embodiment of the invention, constructed as hereinbeforedescribed, the knob 13 may manually be moved in either direction alongthe slit 11 to cause the corresponding variation in the resistanceoffered by the variable resistor unit 14. Let it now be assumed that theknob 13 is moved to the left as viewed in FIGS. 1 and 2. Since thestring 19 is affixed at one point to the pulley 16, or is at leasttightly wrapped around the same, this pulley is caused to turnclockwise, as seen in FIG. 2, as it is pulled by that portion of thestring 19 which extends from the terminal member 13b. Thus, with theleftward movement of the knob 13, the pulley 16 turns clockwise takingup more and more of the string portion extending between itself and theterminal member 13a while gradually releasing the string portion betweenitself and the other terminal member 13b.

The clockwise rotation of the pulley 16 is directly imparted to theshaft 15 of the variable resistor unit 14 on which the pulley ismounted, and the contact arm 30 also affixedly mounted of this shaft 15is thus caused to turn in sliding contact with they resistance element31 which is held stationary. In this manner the electrical resistancebetween the terminals 33 and 35 and that between the terrninals 35and'34 are each varied in proportion to the distance over which the knob13 has been moved. It will be apparent that the contact arm 30 is causedto turn in the opposite direction relative to the resistance element 31if the knob 13 is moved to the right as seen in FIGS. 1 and 2.

Another preferred embodiment of the invention is shown in FIGS. 6 to 11.With particular reference to FIGS. 6 and 7, this second embodiment alsoincludes a panel 10 of substantially rectangular shape having a slit 11extending longitudinally thereof, and a support plate 12 therebelowhaving its tongue 12a rigidly coupled to a knob 13 through the slot 11.A variable resistor unit 14 set forth in relation with the precedingembodiment of the invention is affixed in a similar manner to thesupport plate 12, that is, by a nut 29 meshing with an externally screwthreaded bearing block 28 which rotatably supports the shaft 15 of thevariable resistor unit. A pulley 16 is also screwed at 17 to the shaft15. As previously mentioned, the variable resistor unit 14 may include adustproof housing enclosing the various operative parts thereof. l

The support plate 12 is integrally provided with a pair of arms 40a and40)) which are bent at right angles therewith and which have bores 41aand 41h, FIG. 8, therethrough. Snugly fitted in each of these bores is aslide cylinder 42 illustrated in greater detail in FIG. 10. Preferablyformed of a fluorine resin with its extremely low coefficient offriction, the slide cylinder 42 includes a pair of flanges 43 and 44 ofdifferent diameters on both ends thereof defining a substantial groove45 therebetween. The groove 45 has a width approximately equal to thethickness of the arm 40a or 4011, and the outside diameter of the slidecylinder 42 at this groove 45 is approximately equal to the diameter ofthe bore 41a or 4111. The slide cylinder 42 is hollowed longitudinallytherethrough as indicated at 46 and is further slotted longitudinally at47. A plurality of, three in this embodiment, ridges 48 are formed onits internal surface to minimize the friction with a guide rod 24 whichis to be inserted into the hollow 46.

Due to the provision of the slot 47 the slide cylinder 42 is manuallycompressible to such as extent that at least its flange 44 of smallerdiameter is permitted to pass through the bore 41a or 41b of the arm 40aor 40b. Once inserted into the bore in this manner, each slide cylinder42 is securely retained in position by virtue of the flanges 43 and 44as it regains its initial shape, as will be understood from FIGS. 8 and9. The pair of slide cylinders 42 thus supporting the support plate 12,the variable resistor unit 14 and so forth are slidable on the guide rod24 with minimum frictional resistance.

Referring back to FIGS. 6 and 7, the guide rod 24 has its both endsinserted into and through bores 49a and 49!) formed in the terminalmembers 13a and 13b which in this embodiment of the invention are formedby downwardly bending both ends of the panel 10. The ends of the guiderod 24 projecting out of the bores 49a and 49b are retained by snaprings 50a and 501;, respectively. The guide rod 24 may be greased toassure smoother sliding of the support plate 12 and so forth. I

Since the support plate 12 and the various means affixed thereto mayturn to and fro when supported only by the guide rod 24, the panel issubstantially integrally provided with a downward extension 51 which isfolded at its lower end to provide a guide plate 52 extending parallelto the guide rod 24, as may be best unv derstood from FIG. 7 inparticular. The guide plate 52 is sandwiched between the support plate12 and a leaf spring 53, the latter being screwed at 54 to the former.In order to reduce the frictional resistance which may I be caused bythe guide plate 52, convexities 55 and 56 are formed correspondingly onthe confronting surfaces of the support plate 12 and the leaf spring 53,

such that the guide plate 52 makes substantial point contact therewith.This guide plate may also be greased to further reduce the fricitonalresistance.

As in the preceding embodiments of the invention the pulley 16 screwedat 17 to the shaft of the variable resistor unit 14 is provided with apair of parallel circumferential grooves 18a and 18b to receive the cordor string 19 explained previously in relation with the FIGS. 1 to 5embodiment. The raised border 57 between these parallel grooves isindented at one point thereof as seen in FIG. 7, and the string 19crosses the raised border 57 at this indentation from one of theparallel grooves to the other. Although the string 19 may be affixed tothe pulley l6 in the first described embodiment of the invention, it canalso be retained solely by the opposed edges 58a and 58b of the raisedborder 57 if the spacing between these edges is made sufficiently small.

After making a complete turn around the pulley 16, the string 19 furtherextends crosswise substantially tangentially thereof to have its endtightly affixed to the terminal members 13a and 13b, respectively, in amanner hereinafter described by way ofexample. Both ends of the stringI9 are first inserted into bores formed through member 59a and 59b ofstepped cylindrical shape (shown in the more detail in FIG. 11) and areretained therein as by means of screws 60a and 60b, and these membersare engaged by slots 61a and 61h (also shown in FIG. 11) formed in theterminal members 13a and 13b. It will be seen that the members 5911 and59!; are removably but securely retained in the slots 61a and 6111 bythe steps formed on their circumferences.

The second preferred embodiment of this invention being constructed ashereinbefore set forth, the knob 13 may manually be moved linearly ineither direction along the slit ll of the panel 10 to cause thecorresponding change in the electrical resistance offered by thevariable resistor unit 14 of the construction shown in FIG. 5. With themotion of the knob 13 the support plate 12 and the various means affixedthereto are also caused to move in the same direction as guided by theguide rod 24 and the guide plate 52. The smooth sliding motion of thesupport plate 12 and so forth is ensured since the guide rod 24 is incontact only with the ridges 48 formed on the internal surfaces of theslide cylinders 42 and since the guide plate 52 is in contact only withthe convexities 55 and 56 formed on the confronting surfaces of thesupport plate 12 and the leaf spring 53. Further details of operationofthis embodiment of the invention will be apparent from the foregoingdescription of operation of the FIGS. 1 to 5 embodiment.

FIG. 11 illustrates a modification of the string 19 in the secondembodiment of the invention, in which one or both ends of the string areconnected to one end of a helical tension spring 62 adapted to impartsuitable resiliency thereto. The other end of the spring 62 is insertedinto the member 59a of stepped cylindrical shape as in the FIGS. 6 to 10embodiment and is retained therein by the screw 60a. Also as in thepreceding embodiment the members 59a and 59b are engaged by the slots61a and 61b of the terminal members and 13b. By thus imparting suitableresiliency to the string 19, both ends thereof can be affixed to themembers 59a and 59b before these members are retained in position by theslots 61a and 61!), so that the installation of the string 19 is highlyfacilitated.

Although it is believed that the various objects of this invention,either explicitly stated or implicitly pointed out, are fullyaccomplished by the preferred embodiment herein disclosed, it is also tobe understood that the invention itself is not to be restricted by theexact showing of the accompanying drawings and the description thereof.For instance, the string 19 may be affixed to the terminal member 13aand 13b or to the pulley 16 by means'other than those disclosed herein.Thus, a latitude of modification, substitution and change is intended inthe foregoing disclosure and in some instances some features of theinvention will be employed without the corresponding use of otherfeatures, while still remaining within the spirit and scope of theinvention.

We claim:

1. in combination:

a variable resistor unit comprising a resistance element arranged in asubstantially annular configuration, a contact arm turnable in slidingcontact with said resistance element, and a shaft supporting saidcontact arm;

support means supporting said variable resistor unit in such a mannerthat said shaft is turnable relative to said support means to cause saidcontact arm of said variable resistor unit to turn in sliding contactwith said resistance element;

guide means for directing the linear motion of said support meanstogether with said variable resistor unit over a predetermined stroke;

a pulley fixedly mounted on said shaft of said variable resistor unitfor linear motion with support means;

a string wrapped around said pulley and further extending crosswisesubstantially tangentially thereof to have its end affixed respectivelyto a pair of terminal members positioned adjacent both extremities ofthe stroke of said supportmeans; and

a knob provided to said support means for manually moving the samelinearly in either direction along said guide means whereby said pulleyis turned by said string through an angle proportional to the distancesaid support means is moved, thereby causing corresponding change in theangular position of said contact arm relative to said resistance elementthrough said shaft of said variable resistor unit.

2. The combination as defined in claim 1, wherein said support meansinclude a' support plate of substantially rectangular shape having atongue to which is affixed said knob.

3. The conbination as defined in claim 2, wherein said guide meanscomprise a guide rod slidably supporting said support plate at one endthereof, and a guide plate extending parallel to said guide rod andslidably sandwiched between said support plate and a leaf spring securedto said support plate at the other end thereof.

4. The combination as defined in claim 3, including a pair ofconvexities formed correspondingly on the confronting surfaces of saidsupport plate and said leaf spring to reduce the fricitonal resitancewhich might be caused by said guide plate slidably sandwichedthercbetween.

5. The combination as defined in claim 3, wherein said guide rodslidably supports said support plate through a pair of slide cylindersaffixed to the latter, each of said slide cylinders being hollowed toreceive said guide rod and having a plurality of ridges formedlongitudinally on its internal surface to reduce the frictionalresistance which might be caused by said guide rod.

6. The combination as defined in claim 5, wherein each of said slidecylinders has a pair of flanges of different diameters on both endsthereof and is formed of a resilient material, each of said slidecylinders being further slotted longitudinally so as to be manuallycompressible to such an extent that one of said flanges having a smalldiameter is permitted to pass through a bore formed in said supportplate for installation in place.

7. The combination as defined in claim 5, wherein said slide cylindersare each formed of a fluorine resin having an extremely low coefficientof friction.

8. The combination as defined in claim 1, wherein said guide meansinclude a guide rod slidably supporting said support means.

9. The combination as defined in claim 1, wherein said pulley has a pairof parallel grooves formed circumferentially thereof to receive saidstring, said string being wrapped a single turn around said pulley andcrossing from one of said parallel grooves to the other through anindentation formed in a raised border therebetween.

10. The combination as defined in claim 9, wherein said string isscrewed to said pulley at said indentation formed in the raised borderbetween said parallel grooves.

11. The combination as defined in claim 1, wherein both ends of saidstring are securely retained by a pair of stepped cylindrical members,respectively, said stepped cylindrical members being each removablyengaged by a slot formed in each of said terminal members.

12. The combination as defined in claim 11, including ahelicalcompression spring extending between at least one of said steppedcylindrical members and the corresponding end of said string tofacilitate its installation in place.

mg?" um'n-zp s'rA'rr-zs IA'II'JN'I OFFICE CER'IIFICA'IE OF COIUKECTIONPatent No. 3,800,266 Dated MARCH 26, 1974 C a O I KUNIHfKO MURAO:'I'SUTOMU KAWASAKI It is certified that' error appears in theabove-identified patent and that: said Letters Patent are herebycorrected as shown below:

Front Page, insert [30] Japan 28721/1972 March 9, 1972 Japan 114068/1972September 30, 1973-- Signed and sealed this 17th day of June 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON -Commissioner of Patents AttestingOfficer and Trademarks UNITED STATES lA'l'l'JN'l OFFICE CERTIFICA'JTE 0FCORRECTION Patent: No. 3,300,266 Dated MARCH 26, 1974 I I a O 1 KUNIHfKOMURAO TSUTOMU KAWAAKI It is certified that error appears in theabove-identified patent and that: said Letters Patent are herebycorrected as shown below:

Front Page, insert ----[30] Japan 28721/1972 March 9, 1972 Japan114068/1972 September 30, 1973-- Signed and sealed this 17th day of June1975.

(SEAL) Attest C. MARSHALL DANN RUTH C. MASON Commissioner of PatentsAttesting Officer and Trademarks

1. In combination: a variable resistor unit comprising a resistanceelement arranged in a substantially annular configuration, a contact armturnable in sliding contact with said resistance element, and a shaftsupporting said contact arm; support means supporting said variableresistor unit in such a manner that said shaft is turnable relative tosaid support means to cause said contact arm of said variable resistorunit to turn in sliding contact with said resistance element; guidemeans for directing the linear motion of said support means togetherwith said variable resistor unit over a predetermined stroke; a pulleyfixedly mouted on said shaft of said variable resistor unit for linearmotion with support means; a string wrapped around said pulley andfurther extending crosswise substantially tangentially thereof to haveits end affixed respectively to a pair of terminal members positionedadjacent both extremities of the stroke of said support means; and aknob provided to said support means for manually moving the samelinearly in either direction along said guide means whereby said pulleyis turned by said string through an angle proportional to the distancesaid support means is moved, thereby causing corresponding change in theangular position of said contact arm relative to said resistance elementthrough said shaft of said variable resistor unit.
 2. The combination asdefined in claim 1, wherein said support means include a support plateof substantially rectangular shape having a tongue to which is affixedsaid knob.
 3. The conbination as defined in claim 2, wherein said guidemeans comprise a guide rod slidably supporting said support plate at oneend thereof, anD a guide plate extending parallel to said guide rod andslidably sandwiched between said support plate and a leaf spring securedto said support plate at the other end thereof.
 4. The combination asdefined in claim 3, including a pair of convexities formedcorrespondingly on the confronting surfaces of said support plate andsaid leaf spring to reduce the fricitonal resitance which might becaused by said guide plate slidably sandwiched therebetween.
 5. Thecombination as defined in claim 3, wherein said guide rod slidablysupports said support plate through a pair of slide cylinders affixed tothe latter, each of said slide cylinders being hollowed to receive saidguide rod and having a plurality of ridges formed longitudinally on itsinternal surface to reduce the frictional resistance which might becaused by said guide rod.
 6. The combination as defined in claim 5,wherein each of said slide cylinders has a pair of flanges of differentdiameters on both ends thereof and is formed of a resilient material,each of said slide cylinders being further slotted longitudinally so asto be manually compressible to such an extent that one of said flangeshaving a small diameter is permitted to pass through a bore formed insaid support plate for installation in place.
 7. The combination asdefined in claim 5, wherein said slide cylinders are each formed of afluorine resin having an extremely low coefficient of friction.
 8. Thecombination as defined in claim 1, wherein said guide means include aguide rod slidably supporting said support means.
 9. The combination asdefined in claim 1, wherein said pulley has a pair of parallel groovesformed circumferentially thereof to receive said string, said stringbeing wrapped a single turn around said pulley and crossing from one ofsaid parallel grooves to the other through an indentation formed in araised border therebetween.
 10. The combination as defined in claim 9,wherein said string is screwed to said pulley at said indentation formedin the raised border between said parallel grooves.
 11. The combinationas defined in claim 1, wherein both ends of said string are securelyretained by a pair of stepped cylindrical members, respectively, saidstepped cylindrical members being each removably engaged by a slotformed in each of said terminal members.
 12. The combination as definedin claim 11, including a helical compression spring extending between atleast one of said stepped cylindrical members and the corresponding endof said string to facilitate its installation in place.